Abstract
Bistatic radars, ones in which the transmitter and
receiver are not co-located, have been used since the first deployments of radar systems.
However, the complicated nature of bistatic clutter has so far discouraged the use of
bistatic radar in an airborne system for the purposes of moving target indication (MTI)
with space-time adaptive processing (STAP) for clutter mitigation. In particular, the
separate motion of the transmitter and receiver creates two sources of Doppler for
bistatic clutter. The resulting iso-Doppler relationship can be both complicated and
non-stationary in range and, therefore, difficult to compensate for as is typically done
for monostatic STAP. Yet, for certain applications, operational considerations make a
bistatic MTI radar very attractive. In this paper, we present various considerations for
STAP in an airborne bistatic radar. We first look at the iso-Doppler and iso-bistatic
range relationships in a few illustrative examples that demonstrate the complicated
natureof bistatic clutter and show the strong dependence on the transmitter/receiver
geometry. STAP clutter mitigation is then considered where the range dependence issue is
addressed for the training of the STAP weights. Both Doppler warping, used in monostatic
systems to correct for non-stationarity in range, and a derivative-based weight updating
scheme are investigated. Then, we propose the use of a continuous wave (CW) signal for
bistatic radar. CW signals are possible because of the separate locations of the
transmitter and receiver and have the very attractive characteristic of being unambiguous
in both range and Doppler. In constrast, traditional pulsed waveforms often have
performance limitations due to Doppler and range ambiguities. The STAP processing of a CW
waveform differs from traditional pulse-Doppler STAP used for pulsed waveforms. We outline
the processing steps, including matched filtering, Doppler processing, and STAP, used for
MTI in a CW bistatic radar system. The other advantages and limitations of the CW signal
are discussed in the context of STAP for clutter cancellation.